Over the past century, a variety of different types of lightbulbs have been developed, including incandescent lightbulbs and fluorescent lights. The incandescent bulb is currently the most common type of bulb. In an incandescent bulb, electric current is passed through a metal filament disposed in a vacuum, causing the filament to glow and emit light. The impedance or load characteristic of a standard incandescent bulb is basically resistive in nature.
Recently, bulbs have been developed that produce illumination in a different manner, in particular through the use of light emitting diodes (LEDs). Pre-existing LED lightbulbs have been generally adequate for their intended purposes, but they have not been satisfactory in all respects. As one aspect of this, LED lightbulbs include a driver circuit for the LEDs. These driver circuits are peak charging from the line voltage, and are typically not power factor correcting circuits. LED driver circuits that are not power factor correcting typically have a conduction angle of 30-40 degrees. They exhibit a negative impedance characteristic, rather than a resistive characteristic comparable to that of a standard incandescent bulb.
Dimmers are widely sold today for use with standard incandescent lightbulbs, or in other words bulbs that have a resistive characteristic. Most such dimmers include a triac that produces an output signal, and the output signal is applied to the lightbulb. These dimmers work well with standard incandescent bulbs. However, LED lightbulbs have not worked satisfactorily with these dimmers. More specifically, when used with an LED lightbulb having a negative impedance characteristic (rather than a resistive characteristic), the load applied to the triac is not always sufficient to keep the triac in conduction. Thus, when the triac is supposed to stay on, it will instead tend to oscillate on and off. Further, triacs typically have a resonant frequency, and this resonance can foster the undesired oscillation.
It would be possible to design a dimmer that is not subject to this triac problem. However, for an LED lightbulb to be commercially viable, it is highly desirable that the LED lightbulb be compatible with existing electrical circuits (including those with phase dimmers), so that the LED lightbulb can be readily substituted almost anywhere that a comparable incandescent bulb is used. Electrically, one very simple solution would be to provide a large resistor in the bulb that loads the triac, thereby keeping the triac in conduction, while simultaneously damping its resonance. But as a practical matter, such a resistor would have a physical size that would be too large to be conveniently packaged within the form factor of a typical lightbulb. Further, such a resistor would dissipate a significant amount of power, causing it to be inefficient, and thus relatively expensive to operate. Moreover, the large power dissipation of the resistor would result in the emission of a significant amount of heat. It would be difficult to extract this much heat from the base of a lightbulb. Thus other components (such as integrated circuits) would be heated to temperatures beyond their specifications.